Unit pixel of image sensor and photo detector thereof

ABSTRACT

A unit pixel of an image sensor and a photo detector are disclosed. The photo detector of the present invention configured to absorb light can include: a light-absorbing part configured to absorb light by being formed in a floated structure; an oxide film being in contact with one surface of the light-absorbing part; a source being in contact with one side of the other surface of the oxide film and separated from the light-absorbing part with the oxide film therebetween; a drain facing the source so as to be in contact with the other side of the other surface of the oxide film and separated from the light-absorbing part with the oxide film therebetween; and a channel interposed between the source and the drain and configured to form flow of an electric current between the source and the drain.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention relate to a unit pixel of an imagesensor and a photo detector of the unit pixel.

2. Description of the Related Art

An image sensor is a sensor that transforms an optical signal to anelectrical image signal. When light is irradiated to a light-absorbingpart inside a unit pixel of an image sensor chip, the image sensordetects the light incident at each unit pixel and the amount of thelight and transforms an optical signal to an electrical signal and thentransfers the electrical signal to analog and digital circuits forforming an image.

The conventional image sensors can be classified into CCD (ChargeCoupled Device) types and CMOS (Complementary Metal Oxide Semiconductor)types, based on their structures and operation principles. The CMOS typeimage sensors are commonly referred to as CIS (CMOS Image Sensor).

In the CCD type image sensor, groups of signal electrons generated atthe pixels by the light are transmitted to an output unit by a pulseapplied to a gate, transformed to voltages of the output unit, and sentout one by one.

In the CMOS type image sensor, the signal electrons and holes that aregenerated at the pixels by the light are transformed to voltages insidethe pixels. These voltages are connected to a signal processor,including a row decoder and a column decoder, and sent out of the pixelsby a switching operation according to a clock frequency.

The image sensor can be either an APS (Active Pixel Sensor) or a PPS(Passive Pixel Sensor), according to the presence of an amplifier in theunit pixel.

The PPS is a passive device that does not encompass a signalamplification function inside the pixel, and outputs the electriccurrent of the device to the outside to transform the electric currentto a voltage outside the pixel. On the other hand, the APS is an activedevice that encompasses a signal amplification function inside thepixel.

The PPS is mostly constituted with one photo diode and one selecttransistor, and thus not only can have a greater aperture ratio than theAPS, which requires 3-5 MOS transistors for the same sized pixel, butalso can raise a fill factor related to a light-absorbing efficiency.

However, since the intensity of photoelectric current of the photo diodeis not great and an optical signal is transformed to electric currentthat is vulnerable to an outside environment for use in signalprocessing, the PPS has a problem of fixed pattern noise (FPN).

Therefore, for the same size pixel, the APS can provide an image signalthat has relatively less noise than the PPS, despite the smaller size ofthe light-absorbing part than the PPS, since a multiple number oftransistors are present in the unit pixel.

One electron-hole pair (EHP) is generated for one photon that isincident at a unit pixel light-absorbing part of an image sensor, andthe generated electrons and holes are accumulated in a photo diode,which is a light-absorbing part.

The maximum accumulation electrostatic capacity of a photo diode isproportional to the area of photo detection of the photo diode.Particularly, in the case of CMOS type image sensor, the area in whichthe accompanying transistors are arranged is larger than that of the CCDtype image sensor, and thus increasing the area of the light-absorbingpart is physically limited. Moreover, the photo diode, which is commonlyused as the light-absorbing part of an image sensor, has relativelysmall electrostatic capacity and thus is easily saturated, and it isdifficult to segment the signals generated by the light-absorbing part.

Therefore, the unit pixels of the CMOS image sensor require a relativelylong photoelectric charge accumulation time in order to generate aminimum electric charge for signal processing through the limited photodetection area. Accordingly, it is not easy to manufacture ahigh-density/high-speed frame image sensor by using the unit pixelshaving this kind of light-absorbing part.

The band gap of a silicon semiconductor is 1.12 eV, and a photo detectormade of a silicon semiconductor can detect light energy in wavelengthsof 350 nm to 1150 nm. Here, since the light has different inherentenergy per wavelength and has different depth of penetration when thelight penetrates silicon, which is solid, the photoelectric efficiencyfor each wavelength is also different at the photo detector. In order todetect the wavelengths of visible rays (400-700 nm), the image sensorforms an interface of P-N junction so that a green ray, which commonlyhas energy in the wavelength of 550 nm, can be better detected.Therefore, in the image sensor having this structure, photoelectricefficiencies for a short wavelength, such as blue color, and a longwavelength of a near infrared ray are deteriorated, or the opticalsignal is transformed to a noise.

Prior arts related to an image sensor and a unit pixel of an imagesensor include U.S. Publication Number 2004/0217262 (“UNIT PIXEL IN CMOSIMAGE SENSOR WITH HIGH SENSITIVITY”), U.S. Publication Number2009/0032852 (“CMOS IMAGE SENSOR”) and U.S. Publication Number2010/0073538 (“IMAGE SENSOR”).

U.S. Publication Number 2004/0217262 discloses an image sensor thatincludes one photo diode and four transistors of a transfer transistor,a reset transistor, a drive transistor and a selection transistor andthat inhibits the drive transistor and the selection transistor frombeing affected by leakage of a power supply voltage (VDD) by separatingan active area in which the drive transistor and the selectiontransistor are formed from an active area in which the reset transistoris formed.

However, since U.S. Publication Number 2004/0217262 integrates the photodiode and the four transistors in a limited area, it is difficult toprovide an area of the photo diode for generating a sufficient quantityof electric charge for signal processing.

U.S. Publication Number 2009/0032852 discloses an image sensor that canacquire a wide dynamic range without the loss of sensitivity, byallowing a pixel constituting a CMOS image sensor to have a plurality offloating diffusion regions.

The CMOS image sensor of U.S. Publication Number 2009/0032852 acquires afinal image by acquiring and synthesizing a signal, of which thesensitivity is low but the dynamic range for the brightness is wide, anda signal, of which the dynamic range for the brightness is narrow butthe sensitivity is high, in a separate floating diffusion region.

However, since the above CMOS image sensor acquires the high-sensitivitysignal and the wide dynamic range signal using the respective separatefloating diffusion regions and their related transistors, it isdifficult to provide a sufficient region for a photo detector.

U.S. Publication Number 2010/0073538 discloses an image sensor having ahigh photoconductivity. However, the image sensor of U.S. PublicationNumber 2010/0073538 forms an additional film layer over a PN junction inorder to increase the photoconductivity of a PN junction diode, and thusrequires an additional manufacturing process.

SUMMARY

Contrived to solve the above problems, embodiments of the presentinvention provide a unit pixel of a high-sensitivity/high-performanceimage sensor and a photo detector of the unit pixel that can output agreat photoelectric current with a small quantity of light, realize ahigh-speed frame operation in an environment of low level ofillumination, and record a video ranging from low to high levels ofillumination in a same screen.

An aspect of the present invention features a photo detector configuredto absorb light in a unit pixel of an image sensor transforming theabsorbed light to an electrical signal, which can include: alight-absorbing part configured to absorb light by being formed in afloated structure; an oxide film being in contact with one surface ofthe light-absorbing part; a source being in contact with one side of theother surface of the oxide film and separated from the light-absorbingpart with the oxide film therebetween; a drain facing the source so asto be in contact with the other side of the other surface of the oxidefilm and separated from the light-absorbing part with the oxide filmtherebetween; and a channel interposed between the source and the drainand configured to form flow of an electric current between the sourceand the drain. The light-absorbing part can be doped with first typeimpurities, and the source and the drain can be doped with second typeimpurities. The light-absorbing part can be insulated from the sourceand the drain by the oxide film. Electrons of electron-hole pairsgenerated by the absorbed light can be moved to the source and the drainby a tunneling phenomenon occurred by an electric field concentrated inthe oxide film, and the flow of the electric current of the channel canbe controlled by a change in the quantity of electric charge of thelight-absorbing part caused by the moving of the electrons.

The channel can be formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.

The tunneling phenomenon can be occurred in an area between one of thesource and drain and the light-absorbing part.

The source and the drain can be formed by doping the second typeimpurities in a body, and the body can be floated.

A threshold voltage of the photo detector can be changed due to thetunneling phenomenon occurred in the oxide film.

The photo detector can also include a light-blocking layer configured toblock absorption of light in areas other than an upper surface of thelight-absorbing part.

Another aspect of the present invention features a unit pixel of animage sensor configured to transform absorbed light to an electricalsignal, which can include: a photo detector configured to cause anelectric current to flow using a change in the quantity of electriccharge caused by incident light; and a select device configured tooutput the electric current generated by the photo detector to a unitpixel output terminal. The photo detector can include: a light-absorbingpart formed in a floated structure and configured to absorbed light; anoxide film being in contact with one surface of the light-absorbingpart; a source being in contact with one side of the other surface ofthe oxide film and separated from the light-absorbing part with theoxide film therebetween; a drain facing the source so as to be incontact with the other side of the other surface of the oxide film andseparated from the light-absorbing part with the oxide filmtherebetween; and a channel formed between the source and the drain andconfigured to form flow of an electric current between the source anddrain. The select device can include: a drain being connected with thesource of the photo detector; a source being accessed to the unit pixeloutput terminal; and a gate configured to receive a control signal froman outside, and a switching operation can be performed based on thecontrol signal. The light-absorbing part can be doped with first typeimpurities, and the source and the drain the photo detector can be dopedwith second type impurities. The light-absorbing part can be insulatedfrom the source and the drain of the light-absorbing part by the oxidefilm. Electrons of electron-hole pairs generated by the absorbed lightcan be moved to the source and the drain by a tunneling phenomenonoccurred by an electric field concentrated in the oxide film, and theflow of the electric current of the channel can be controlled by achange in the quantity of electric charge of the light-absorbing partcaused by the moving of the electrons.

The photo detector can be realized in an LDD (light doped drain)structure.

The source of the photo detector and the drain of the select device canbe formed in a same active area.

Yet another aspect of the present invention features a photo detectorconfigured to absorb light in a unit pixel of an image sensor forconverting the absorbed light to an electrical signal, which caninclude: a light-absorbing part configured to absorb light by beingformed in a floated structure; an oxide film being in contact with onesurface of the light-absorbing part; a control terminal formed on theother surface of the light-absorbing part and configured to transfer areset signal to the light-absorbing part; a source being in contact withone side of the other surface of the oxide film and separated from thelight-absorbing part with the oxide film therebetween; a drain facingthe source so as to be in contact with the other side of the othersurface of the oxide film and separated from the light-absorbing partwith the oxide film therebetween; and a channel interposed between thesource and the drain and configured to form flow of an electric currentbetween the source and the drain. The light-absorbing part can be dopedwith first type impurities, and the source and the drain are doped withsecond type impurities. The light-absorbing part can be insulated fromthe source and the drain by the oxide film. Electrons of electron-holepairs generated by the absorbed light can be moved to the source and thedrain by a tunneling phenomenon occurred by an electric fieldconcentrated in the oxide film, and the flow of the electric current ofthe channel can be controlled by a change in the quantity of electriccharge of the light-absorbing part caused by the moving of theelectrons. The control terminal can move remaining electric charge bytransmitting the reset signal to the light-absorbing part.

The channel can be formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.

The tunneling phenomenon can be occurred in an area between one of thesource and drain and the light-absorbing part.

The source and the drain can be formed by doping the second typeimpurities in a body, and the body can be floated.

A threshold voltage of the photo detector can be changed due to thetunneling phenomenon occurred in the oxide film.

The photo detector can also include a light-blocking layer configured toblock absorption of light in areas other than an upper surface of thelight-absorbing part.

Still another aspect of the present invention features a unit pixel ofan image sensor configured to transform absorbed light to an electricalsignal, which can include: a photo detector configured to cause anelectric current to flow using a change in the quantity of electriccharge caused by incident light; and a select device configured tooutput the electric current generated by the photo detector to a unitpixel output terminal. The photo detector can include: a light-absorbingpart configured to absorb light by being formed in a floated structure;an oxide film being in contact with one surface of the light-absorbingpart; a control terminal formed on the other surface of thelight-absorbing part and configured to transfer a reset signal to thelight-absorbing part; a source being in contact with one side of theother surface of the oxide film and separated from the light-absorbingpart with the oxide film therebetween; a drain facing the source so asto be in contact with the other side of the other surface of the oxidefilm and separated from the light-absorbing part with the oxide filmtherebetween; and a channel interposed between the source and the drainand configured to form flow of an electric current between the sourceand the drain. The select device can include: a drain being connectedwith the source of the photo detector; a source being accessed to theunit pixel output terminal; and a gate configured to receive a controlsignal from an outside, and a switching operation can be performed basedon the control signal. The light-absorbing part can be doped with firsttype impurities, and the source and the drain the photo detector can bedoped with second type impurities. The light-absorbing part can beinsulated from the source and the drain of the light-absorbing part bythe oxide film. Electrons of electron-hole pairs generated by theabsorbed light can be moved to the source and the drain by a tunnelingphenomenon occurred by an electric field concentrated in the oxide film,and the flow of the electric current of the channel is controlled by achange in the quantity of electric charge of the light-absorbing partcaused by the moving of the electrons. The control terminal can moveremaining electric charge by transmitting the reset signal to thelight-absorbing part.

The photo detector can be realized in an LDD (light doped drain)structure.

The source of the photo detector and the drain of the select device canbe formed in a same active area.

The unit pixel can also include a second photo detector seriallyconnected with the photo detector and the select device and formed in asame structure as the photo detector.

The unit pixel can also include a second photo detector connected inparallel with the photo detector and formed in a same structure as thephoto detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a tunnel junction photo detector inaccordance with a first embodiment of the present invention.

FIG. 2 is another perspective view of the tunnel junction photo detectorin accordance with the first embodiment of the present invention.

FIG. 3 is a cross-sectional view of the tunnel junction photo detectorin accordance with the first embodiment of the present invention.

FIG. 4 is a cross-sectional view for illustrating the forming of achannel of the tunnel junction photo detector in accordance with thefirst embodiment of the present invention.

FIG. 5 is a cross-sectional view for illustrating a light-blockingmethod of the tunnel junction photo detector in accordance with thefirst embodiment of the present invention.

FIG. 6 is a cross-sectional view for illustrating an incident angle oflight of the tunnel junction photo detector in accordance with the firstembodiment of the present invention.

FIG. 7 is a circuit schematic of a unit pixel using the tunnel junctionphoto detector in accordance with the first embodiment of the presentinvention.

FIG. 8 is a cross-sectional view of the unit pixel using the tunneljunction photo detector in accordance with the first embodiment of thepresent invention.

FIG. 9 is a perspective view of a tunnel junction photo detector inaccordance with a second embodiment of the present invention.

FIG. 10 is a cross-sectional view of the tunnel junction photo detectorin accordance with the second embodiment of the present invention.

FIG. 11 is a circuit schematic of a unit pixel in which two tunneljunction photo detectors are serially connected in accordance with athird embodiment of the present invention.

FIG. 12 is a circuit schematic of a unit pixel in which two tunneljunction photo detectors are connected in parallel in accordance with afourth embodiment of the present invention.

FIG. 13 is a perspective view of a tunnel junction photo detector inaccordance with a fifth embodiment of the present invention.

FIG. 14 is a cross-sectional view of the tunnel junction photo detectorin accordance with the fifth embodiment of the present invention.

DETAILED DESCRIPTION

Since there can be a variety of permutations and embodiments of thepresent invention, certain embodiments will be illustrated and describedwith reference to the accompanying drawings. This, however, is by nomeans to restrict the present invention to certain embodiments, andshall be construed as including all permutations, equivalents andsubstitutes covered by the ideas and scope of the present invention.

Throughout the description of the present invention, when describing acertain technology is determined to evade the point of the presentinvention, the pertinent detailed description will be omitted. Termssuch as “first” and “second” can be used in describing various elements,but the above elements shall not be restricted to the above terms. Theabove terms are used only to distinguish one element from the other.

When one element is described as being “connected” or “accessed” toanother element, it shall be construed as being connected or accessed tothe other element directly but also as possibly having another elementin between. On the other hand, if one element is described as being“directly connected” or “directly accessed” to another element, it shallbe construed that there is no other element in between.

Hereinafter, certain embodiments of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a perspective view of a tunnel junction photo detector inaccordance with a first embodiment of the present invention. Asillustrated in FIG. 1, a photo detector of a unit pixel is realizedusing a tunnel junction instead of the conventional photo diode. Here, atunnel junction device, in which a thin insulation layer is joined inbetween two conductors or semiconductors, refers to a device thatoperates using a tunneling effect generated in the insulation layer. Forreference, the tunneling effect is a quantum mechanical phenomenon inwhich a particle passes through an area having a greater potentialenergy than its inherent dynamic energy under a strong electric field.

In an embodiment of the present invention, the photo detector of a unitpixel can be generated using said tunnel junction device, and the“tunnel junction photo detector” in the description and claims of thepresent invention refers to a photo detector realized using said tunneljunction device. The tunnel junction photo detector can be realizedusing various kinds of structures, for example, the general n-MOSFET orp-MOSFET structure. Also, in addition to MOSFET, the unit pixel can berealized using an electronic device having a structure that can providea tunneling effect, for example, JFET, HEMT, etc.

In FIG. 1, a tunnel junction photo detector 100 is realized in an NMOSstructure. The tunnel junction photo detector 100 is formed on a p-typesubstrate 110 and includes an N+ diffusion layer 120 corresponding to asource and an N+ diffusion layer 130 corresponding to a drain in ageneral NMOS electronic device. Hereinafter, the N+ diffusion layers120, 130 will be referred to as the “source” and “drain” in the tunneljunction photo detector, respectively.

Formed in between the source 120 and the drain 130 is a thin oxide film140, and formed above the oxide film 140 is a poly-silicon 150, in whicha p-type impurity is doped, corresponding to the gate in the NMOSstructure. Here, in order to facilitate the tunneling phenomenon, it ispreferable that the oxide film 140 is formed in the thickness of 10 nmor less, for example, 2 nm, 5 nm, 7 nm, etc.

Unlike a gate in a general NMOS electronic device, the poly-silicon 150is formed in a floated structure. In addition, the poly-silicon 150 doesnot form a silicide layer above the poly-silicon 150 and operates as anarea that absorbs light. If a silicide layer is formed over thepoly-silicon 150, metallic impurities make it difficult forelectron-hole pairs to be formed by the light and for the light topermeate into the poly-silicon 150 because the incident light isreflected.

Hereinafter, an area of the poly-silicon 150 of the tunnel junctionphoto detector 100 in the description and claims of the presentinvention will be referred to as a “light-absorbing part.”

Formed above the source 120 and the drain 130 are metal contacts 121,131 that are respectively connected with outside nodes. The metalcontact 121 of the source 120 is connected with an outside through ametal line 122, and the metal contact 131 of the drain 130 is likewiseconnected with an outside through a metal line 132.

The tunnel junction photo detector 100 is formed in a structure in whichthe p-type substrate 110 is floated, unlike the general NMOS electronicdevice. Accordingly, the tunnel junction photo detector 100 is differentin structure from the general NMOS electronic device in that only thesource 120 and the drain 130 are connected with the outside nodes.

Moreover, the tunnel junction photo detector 100 can be formedsymmetrically. Accordingly, it is possible that the source 120 and thedrain 130 are substituted with each other.

An upper part of the photo detector 100 excluding an upper surface ofthe light-absorbing part 150 has a light-blocking layer 180 formedthereon. Referring to FIG. 5, the light-blocking layer 180 blocks thelight from being absorbed in areas other than the light-absorbing part150, by being formed on an upper part of the tunnel junction photodetector 100 excluding the upper surface of the light-absorbing part150. This is to efficiently tunnel photoelectric charges of thelight-absorbing part 150. Moreover, this is to inhibit parasiticelectric charges from being generated by the absorption of light inareas other than the light-absorbing part 150 as well as to obtaincontrolled photoelectric current. The light-blocking layer 180 can beformed through a silicide process and can be prevented from being formedover the light-absorbing part 150 through the use of a mask.

FIG. 2 illustrates a photo detector having a micro lens.

In FIG. 2, a micro lens 170 converges light incident to the photodetector 100. In a common image sensor, the light is incident at theimage sensor through an optical lens (not shown). The light havingpassed the optical lens arrives at the micro lens 170 located above thephoto detector 100. The micro lens 170 converges the light incident thelight incident at a front surface of the unit pixel and allows theincident light to enter an upper surface 151 of the light-absorbing part150. Here, the upper surface 151 of the light-absorbing part 150 can bedirectly exposed, or a passivation layer, through which light canreadily permeate, can be formed in between the light-absorbing part 150and the air. The micro lens 170 is arranged above the light-absorbingpart 150 where the light-blocking layer 180 is not formed in such a waythat the light is converged.

An electric field is formed between the source 120 and drain 130 and thelight-absorbing part 150 by the incident light, and a channel 160 isformed between the source 120 and the drain 130. Specifically,electron-hole pairs are generated by the light incident at thelight-absorbing part 150, and electrons of the generated electron-holepairs are moved to the source 120 or the drain 130 from thelight-absorbing part 150 by a tunneling effect. Due to the loss of theelectrons, the quantity of electric charge in the holes of thelight-absorbing part 150 becomes relatively increased. Accordingly,unlike a common NMOS device, the channel 160 is formed and electriccurrent becomes to flow between the source 120 and the drain 130 due tothe effect of threshold voltage modulation caused by a change in thequantity of electric charge of the light-absorbing part 150.

Meanwhile, the tunnel junction photo detector 100 can be realized in anLDD (light doped drain) structure. By realizing the tunnel junctionphoto detector in an LDD structure, it becomes possible to decrease thegeneration of a hot carrier caused by a short channel effect. FIG. 3shows a cross-sectional view of the tunnel junction photo detectorformed in an LDD structure in accordance with an embodiment of thepresent invention.

In FIG. 3, the tunnel junction photo detector 100 is formed on thep-type substrate 100 and includes the source 120 and the drain 130, bothof which are N+ diffusion layers. Here, the source 120 and the drain 130are symmetrical to each other and can have identical device properties.An LDD area 123, which is an n-type area that is lightly doped, isformed in an area that is adjacent to the source 120 and the oxide film141. Moreover, an LDD area 123, which is an n-type area that is lightlydoped, can be formed in an area that is adjacent to the drain 130 andthe oxide film 140. The light-absorbing part 150 can be formed to havethe same length as the distance between the LDD area 123 of the source120 and the LDD area 133 of the drain 130.

When light having greater energy than energy to which doped impuritiesare coupled is irradiated to the light-absorbing part 150, light havinggreater energy than coupling energy of the doped holes is incident atthe light-absorbing part 150, which is poly-silicon in which p-typeimpurities are doped, the plurality of holes formed by doping theimpurities become a free state within a boundary defined by the oxidefilm 140, which prevents the electric charge from moving in anequilibrium state. Here, the generated electron-hole pairs are presentin the states of electrons and holes for a predetermined duration untilthey are recombined, increasing the number of holes locally and thusincreasing the quantity of electric charge.

The separated electrons freely move outside a grain boundary of thepoly-silicon. Here, if an outside voltage is supplied to the drain 130,the electrons are pulled to near an edge of the LDD area 133 of thedrain. Accordingly, the electrons are accumulated near the edge of thelight-absorbing part 150 that is adjacent to the LDD area 133 andreceive the electric field. The electric field that is relativelystronger is formed as the number of integrated electrons increases.Accordingly, the phenomenon of integration of electrons near the edge ofthe light-absorbing part 150 becomes accelerated. The more intense thelight irradiated to the light-absorbing part 150 is, the moreelectron-hole pairs are formed and the greater electric field is formed.

The tunneling phenomenon occurs readily at a boundary area 141 where thedistance between the LDD areas 123 and the light-absorbing part 150 isthe shortest and at a boundary area 142 where the distance between theLDD areas 133 and the light-absorbing part 150 is the shortest. Atunneling effect occurs while energy level conditions are satisfied inthe boundary areas 141, 142. By the tunneling effect, the electronsintegrated in the boundary areas 141, 142 of the light-absorbing part150 can be moved to the source 120 or the drain 130. In such a case, thetotal quantity of electric charge of the light-absorbing part 150 ischanged. That is, the quantity of electric charge of the holes isincreased by as much as the number of electrons lost by the tunnelingeffect, and the channel 160 is formed between the source 120 and thedrain 130 due to the effect of threshold voltage modulation caused by achange of potential of the light-absorbing part 150. The quantity ofelectric current is increased through the formed channel 160.

Meanwhile, if the intensity of light becomes smaller or the light isblocked, the quantity of electric charge returns to its original state,in an opposite way to the above phenomenon. In case the light isintensely irradiated and then blocked, the light-absorbing part 150becomes to have a quantity of weak (+) electric charge due to theincrease in the quantity of electrons, but an electric field is formedby the electrons accumulated in the boundary area 142 of the LDD area133 of the drain and the boundary area 141 of the LDD area 123 of thesource, in which electric potentials are relatively low. Afterwards, thetunneling effect occurs in the boundary areas 141, 142 in directions theelectrons flowing into the light-absorbing part 150. When the electronsflowed in by the tunneling effect are recombined with the holes, thequantity of (+) electric charge becomes decreased. This will weaken theelectric field by the light-absorbing part 150, and reduce or eliminatethe channel 160 between the source 120 and the drain 130. Accordingly,the electric current flowing through the channel 160 stops flowing.

The channel 160 is designed in a manufacturing process of the tunneljunction photo detector 100 in such a way that the channel 160 is in astate immediately before pinch-off. FIG. 4 shows the channel 160 of thepresent invention. In FIG. 4, the channel 160 is generated by a voltagedifference between the source 120 and the drain 130. Moreover, adepletion layer 161 is formed around the source 120, the drain 130 andthe channel 160 due to the supplied voltage. The channel 160 ismanufactured by adjusting a W/L ratio, which is a ratio between itswidth and length, in the manufacturing process so that the channel 160is in the state immediately before pinch-off while no outside voltage issupplied to the source 120 and the drain 130. Here, the W/L can bedesigned experimentally for each manufacturing process of the tunneljunction photo detector since the conditions in which pinch-off occurscan be different for each doping concentration of an element and eachproperty of the tunnel junction photo detector.

The tunneling phenomenon occurs continuously in the boundary area 141,142 between the LDD areas of the source 120 and drain 130 and thelight-absorbing part 150. However, tunneling is more prominent in theside of the drain 130 when the intensity of light is greater, and in theside of the source 120 when the intensity of light is smaller, therebymaintaining the state of equilibrium.

FIG. 6 is a cross-sectional view for illustrating an incident angle oflight of a tunnel junction photo detector in accordance with a firstembodiment of the present invention.

In FIG. 6, the light convergeed through the micro lens is incident tothe light-absorbing part 150 along a light incident path having apredetermined slope by multiple layers of shades 192. The shades 192 canbe formed by appropriately arranging metal lines for signal transfer anddevice control along the incident path. Formed in between the multiplelayers of shades 192 can be passivation layers 182, which can be formedwith a material that has little reflection of the incident light.

Through the above structure of tunnel junction photo detector, itbecomes possible to flow photoelectric currents that are hundreds tothousands times greater than the conventional photo diode. While theconventional photo diode distinguishes the brightness only by thequantity of electric charge accumulated in the electrostatic capacity,the change in the quantity of electric charge of the light-absorbingpart caused by the light works as the electric field effect in the photodetector in accordance with an embodiment of the present invention,thereby controlling the electric current flow of the channel Moreover,since the required electric charge can be infinitely supplied throughthe drain, a signal can be self-amplified in the photo detector.Therefore, it is possible to realize a unit pixel in a PPS structure,without introducing an additional signal amplification device. Ofcourse, it is also possible to realize a unit pixel using theconventional APS method. In the present embodiment, however, the unitpixel is realized in the PPS structure using the tunnel junction photodetector, for the convenience of description and understanding.

Hereinafter, some embodiments of a unit pixel of an image sensorrealized using the tunnel junction photo detector in accordance with theabove embodiments will be described with reference to the accompanyingdrawings.

FIG. 7 is a circuit schematic of a unit pixel using the tunnel junctionphoto detector in accordance with the first embodiment of the presentinvention. The unit pixel shown in FIG. 7 includes one tunnel junctionphoto detector 100 and one select transistor 600.

Here, the one select transistor can be formed of various devices, forexample, the conventional MOSFET structure. In this case, the tunneljunction photo detector and the select transistor can be simultaneouslyrealized using a manufacturing process of the conventional MOSFET,simplifying the manufacturing process and saving the cost.

The drain 130 of the tunnel junction photo detector 100 is accessed to apower supply voltage (VDD), and the source 120 is connected to a drain630 of the select transistor 600.

Although the source 120 and drain 130 of the tunnel junction photodetector 100 are symmetrical and identical to each other, thedescription and claims of the present specification will refer the drainas an area accessed to the power supply voltage (VDD) or an outsideelectric charge supply.

The light-absorbing part 150 of the tunnel junction photo detector 100is formed in a floating gate structure that is restricted to allow lightto incident at a gate only. The light-absorbing part 150 does not havemetal silicide formed on an upper surface thereof, and thus it ispossible to absorb the light through the light-absorbing part 150. Ap-type substrate (P-sub), which corresponds to a body of a common NMOSstructure, can be also formed in a floated structure. Therefore, thetunnel junction photo detector 100 is connected electrically with anoutside node through the source 120 and the drain 130.

In the present embodiment, the select transistor 600 can be constitutedwith NMOS. The drain 630 of the select transistor 600 is connected tothe source 120 of the tunnel junction photo detector 100, and a source620 is connected to a unit pixel output terminal (“I_output”). A controlsignal (“Sx”) for the control of on-off of the select transistor 600 canbe supplied through a gate 650.

Moreover, a body 610 of the select transistor 600 can be formed in afloated structure, like the tunnel junction photo detector 100. This isfloating a body 110 of the tunnel junction photo detector 100. In such acase, in the gate control of the select transistor 600 that isswitch-operated, its switching function can be maintained by supplying aslightly higher voltage than the power supply voltage (VDD).

FIG. 8 is a cross-sectional view of a unit pixel constituted with thetunnel junction photo detector and the select transistor in an NMOSstructure in accordance with the first embodiment of the presentinvention.

As illustrated in FIG. 8, both the tunnel junction photo detector 100and the select transistor 600 can be formed in a floated structurehaving the same P-sub as the body. In this case, the source 120 of thetunnel junction photo detector 100 and the drain 630 of the selecttransistor 600 can be formed in a same active area, simplifying thestructure and reducing the size of the unit pixel.

Hereinafter, a tunnel junction photo detector in accordance with asecond embodiment of the present invention will be described. In FIG. 9,a tunnel junction photo detector 200 in accordance with a secondembodiment of the present invention is realized in a PMOS structure. AnN-well 215 is formed by injecting n-type impurities in a p-typesubstrate 210. A source 220 and a drain 230 are formed by injectinghigh-concentration p-type impurities in the formed N-well 215. A thinoxide film 240 is formed between the source 220 and the drain 230, andformed above the oxide film 240 is a light-absorbing part 250, which isformed in a floated structure. The light-absorbing part 250 can bepoly-silicon in which n-type impurities are doped.

Like the first embodiment, the light-absorbing part 250 does not have ametallic silicide layer formed on an upper surface thereof, andfunctions as an area that absorbs light.

Formed above the source 220 and the drain 230 are metal contacts 221,231 that are respectively connected with outside nodes. The metalcontact 221 of the source 220 is connected with an outside through ametal line 222, and the metal contact 231 of the drain 230 is likewiseconnected with an outside through a metal line 232.

The N-well 215 is formed in a floated structure. Therefore, the tunneljunction photo detector is structurally different from a general PMOSelectronic device in that only the source 220 and the drain 230 areconnected with outside nodes.

An upper part of the photo detector 200 excluding the light-absorbingpart 250 has a light-blocking layer 280 formed thereon. Thelight-blocking layer 280 blocks the light from being absorbed in areasother than the light-absorbing part 250.

A micro lens 270 converges the light incident at the tunnel junctionphoto detector 200 and guides the incident light to the light-absorbingpart 250.

An electric field is formed between the source 220 and drain 230 and thelight-absorbing part 250 by the incident light, and a channel 260 isformed between the source 220 and the drain 230.

Like the first embodiment, the tunnel junction photo detector 200 of thesecond embodiment can be realized in an LDD (light doped drain)structure. In this case, a tunneling phenomenon can occur readily at aboundary area 241 where the distance between an LDD area 223 and thelight-absorbing part 250 is the shortest and at a boundary area 242where the distance between an LDD area 233 and the light-absorbing part250 is the shortest. A tunneling effect occurs while energy levelconditions are satisfied in the boundary areas 241, 242. By realizingthe tunnel junction photo detector in the above LDD structure,generation of hot carriers caused by a short channel effect can bereduced when a photoelectric current is generated by optical absorption,like in a common electronic device.

The tunnel junction photo detectors 100, 200 described above have asensitivity property that is superbly higher than the conventional photodiode, and thus it is possible to manufacture a unit pixel that uses amuch smaller area. By using this advantage, it is possible to integratea plurality of tunnel junction photo detectors in a unit pixel.

FIG. 11 is a circuit schematic of a unit pixel in which two tunneljunction photo detectors are serially connected in accordance with athird embodiment of the present invention, and FIG. 12 is a circuitschematic of a unit pixel in which two tunnel junction photo detectorsare connected in parallel in accordance with a fourth embodiment of thepresent invention.

In FIG. 11, two tunnel junction photo detectors 300-1, 300-2 areserially connected. In this case, the current flowing through the selecttransistor 600 to an output terminal (“I_output”) is increased by nearlytwice. Therefore, an ultrahigh sensitivity effect, which can render aclear video in a low level of illumination of 0.1 lux or lower, isgenerated without adding an additional amplification circuit.

In FIG. 12, two tunnel junction photo detectors 400-1, 400-2 areconnected in parallel. In this case, the voltage between the tunneljunction photo detectors 400-1, 400-2 connected parallel becomes twicegreater. Moreover, it becomes possible to use one of the parallelconnected tunnel junction photo detectors through a control of anoperation circuit. Therefore, a dynamic range of the unit pixel can beselectively controlled, and a bright area and a dark area can berendered in a much more improved manner.

Hereinafter, a tunnel junction photo detector in accordance with a fifthembodiment of the present invention will be described.

In a tunnel junction photo detector 500 shown in FIG. 13, which has asimilar structure as the tunnel junction photo detector 100 inaccordance with the first embodiment of the present invention, a source520, a drain 530 and a thin oxide layer 540, and a light-absorbing part550, on which a light-blocking layer 580 is not formed, is formed abovethe oxide layer 540. The tunnel junction photo detector 500 alsoincludes a select device 590, which is made of poly-silicon, on an uppersurface of the light-absorbing part 500. The select device 590 can beformed to be in contact with an upper surface of the light-absorbingpart 550, and an insulation layer can be interposed between the selectdevice 590 and the light-absorbing part 550.

By supplying a predetermined voltage (e.g., 1.8V or 2.5V) to thelight-absorbing part 550 through the select device 590, an electriccharge remaining in the light-absorbing part 550 can be diffused ortunneled to the oxide layer 540. Resetting the remaining electric chargeusing this structure can fundamentally prevent an image lag from beinggenerated and can provide an effect that is similar to an electronicshutter that can operate an image sensor in high speed more effectively.Here, if the select device 590 is formed on the upper surface of thelight-absorbing part 550, the width of the light-absorbing part 550excluding the area in which the select device 590 is formed needs to begreater than a wavelength of the light to be absorbed.

The select device 590 can be realized to be in contact with a locationthat is not the upper surface of the light-absorbing part 550, whichabsorbs the light. In FIG. 14, portions of the upper surface of thelight-absorbing part 550 are configured to be blocked from an outside byan insulation layer or a light-blocking film 580. Here, the selectdevice 590 can be formed to be in contact with a location of thelight-absorbing part 550 that is covered. In this case, the area of thelight-absorbing part 550 that absorbs the light can be amply provided.

Hitherto, the unit pixel of an image sensor as well as the tunneljunction photo detector of the unit pixel having the technical featuresof the present invention have been described through the aboveembodiments.

Through the above structure, it is possible that the unit pixel of thepresent invention allows hundreds to thousands times greaterphotoelectric currents than the conventional photo diode for a givenquantity of light. This is because, unlike the conventional photo diodein which contrast is distinguished by the quantity of electric chargeaccumulated in the electrostatic capacity only, the present inventioncontrols the electric current flow of the source-drain channels owing tothe electric field effect from the change in the quantity of electriccharge of the floating gate and at the same time generates an effect ofself-amplification owing to infinite supply of electric charges throughthe drain.

The unit pixel and the tunnel junction photo detector described in theabove embodiments can be realized in a PPS type, which does not need tohave a separate amplification device inside the unit pixel, unlike theconventional CIS.

Moreover, through the above structure, it is possible to realize ahigh-sensitivity/high-speed image sensor through the standard CMOSprocess.

Since there is little or no parasitic capacitor compared to the outputcurrent of the photo detector inside the pixel, the image sensor havingthe configuration described above does not require any integrationaction until a pixel is selected by a row decoder. Therefore, it becomespossible to develop a high-speed frame image sensor by multi-processinga signal in a modified rolling shutter method as well.

Since the unit pixel has a very simple structure and is not big,ultrahigh-speed images of 500-10,000 fps can be realized by forming acapacitor inside the unit pixel like the global shutter method, storingdata simultaneously in an analog memory and reading the data in highspeed.

The above description has been provided in illustrative purposes only,and it shall be appreciated that it is possible for any ordinarilyskilled person in the art to which the present invention pertains toeasily modify the present invention without departing the technicalideas and essential features of the present invention. As used herein,the term “aspect” may be used interchangeably with the term“embodiment.”

Therefore, it shall be appreciated that the embodiments described aboveare illustrative, not restrictive. For instance, any elements describedto be combined can be also embodied by being separated, and likewise,any elements described to be separated can be also embodied by beingcombined.

The scope of the present invention shall be defined not by the abovedescription but rather by the claims appended below, and it shall beunderstood that all possible permutations or modifications that can becontrived from the meanings, scopes and equivalents of the claims areincluded in the scope of the present invention.

What is claimed is:
 1. A photo detector configured to absorb light in aunit pixel of an image sensor transforming the absorbed light to anelectrical signal, comprising: a light-absorbing part configured toabsorb light by being formed in a floated structure; an oxide film beingin contact with one surface of the light-absorbing part; a source beingin contact with one side of the other surface of the oxide film andseparated from the light-absorbing part with the oxide filmtherebetween; a drain facing the source so as to be in contact with theother side of the other surface of the oxide film and separated from thelight-absorbing part with the oxide film therebetween; and a channelinterposed between the source and the drain and configured to form flowof an electric current between the source and the drain, wherein thelight-absorbing part is doped with first type impurities, and the sourceand the drain are doped with second type impurities, wherein thelight-absorbing part is insulated from the source and the drain by theoxide film, wherein electrons of electron-hole pairs generated by theabsorbed light are moved to the source and the drain by a tunnelingphenomenon occurred by an electric field concentrated in the oxide film,and the flow of the electric current of the channel is controlled by achange in the quantity of electric charge of the light-absorbing partcaused by the moving of the electrons.
 2. The photo detector of claim 1,wherein the channel is formed in a state immediately before pinch-off byadjusting a W/L ratio, which is a ratio between a width (W) and a length(L) of the channel.
 3. The photo detector of claim 1, wherein thetunneling phenomenon is occurred in an area between one of the sourceand drain and the light-absorbing part.
 4. The photo detector of claim1, wherein the source and the drain are formed by doping the second typeimpurities in a body, and the body is floated.
 5. The photo detector ofclaim 1, wherein a threshold voltage of the photo detector is changeddue to the tunneling phenomenon occurred in the oxide film.
 6. The photodetector of claim 1, further comprising a light-blocking layerconfigured to block absorption of light in areas other than an uppersurface of the light-absorbing part.